1. Field of the Invention
The present invention relates to a magnetoresistance effect film for reading the magnetic field intensity of a magnetic recording medium or the like as a signal and, in particular, to a ferromagnetic tunnel magnetoresistance effect element which is capable of reading a small magnetic field change as a greater electrical resistance change signal. The ferromagnetic tunnel magnetoresistance effect element is mainly incorporated in, for example, a hard disk drive so as to be used.
2. Description of the Prior Art
Following the high densification of hard disks, highly sensitive magnetic heads with high outputs have been demanded. In response to these demands, attention has been paid to a ferromagnetic tunnel magnetoresistance effect element having a multilayered structure in the form of ferromagnetic layer/tunnel barrier layer/ferromagnetic layer so as to utilize a ferromagnetic tunnel magnetoresistance effect.
The ferromagnetic tunnel magnetoresistance effect is a phenomenon that when a current is applied in a laminate direction between a pair of ferromagnetic layers which sandwich a tunnel barrier layer, a tunnel current flowing in the tunnel barrier layer changes depending on a relative angle of magnetization between both ferromagnetic layers.
In this case, the tunnel barrier layer is a thin insulation film which allows electrons to pass therethrough while keeping spin due to the tunnel magnetoresistance effect. When a relative angle of magnetization between both ferromagnetic layers is small, the tunnel probability is increased and, therefore, a resistance of the current flowing between both ferromagnetic layers is decreased. In contrast with this, when a relative angle of magnetization between both ferromagnetic layers is large, the tunnel probability is lowered so that a resistance of the current flowing therebetween is increased.
The ferromagnetic tunnel magnetoresistance effect element (hereinafter simply referred to as "TMR element") has been prepared in the following manner: A lower ferromagnetic layer is formed on a substrate. Then, after forming a non-magnetic layer of typically Al on the lower ferromagnetic layer, the non-magnetic layer (Al) is oxidized according to the natural oxidation method, the plasma oxidation method or the like so as to complete a tunnel barrier layer. Thereafter, an upper ferromagnetic layer is formed on the tunnel barrier layer.
When applying the TMR element to an HDD head, it is essential to lower the electrical resistance of the element. The reason is as follows: Specifically, the resistance of a TMR element is basically expressed by the following equation (1): EQU R.sigma.=C.sigma. exp (-2.kappa.d) (1)
.kappa.=(2 m.phi./h.sup.2).sup.1/2 PA1 wherein d represents a thickness of a barrier, .phi. represents a magnitude of a barrier potential measured from the Fermi level, and C.sigma. represents an amount determined by an electron state of magnetic layers and an insulation layer and may be considered approximately proportional to the product of the Fermi levels of the two magnetic layers.
According to the foregoing equation (1), it is understood that the lower resistance of the element can be achieved by reducing the thickness d of the barrier or reducing the barrier potential .phi.. However, it is not preferable to reduce the barrier potential .phi. because the tunnel current itself is decreased to result in reduction of the TMR effect.
On the other hand, it has been reported that even in case of approximately the same barrier thicknesses d, a junction resistance largely changes due to a difference in forming method of a tunnel barrier layer (The 43rd MMM Conference 1998, GA-03). Specifically, it has been reported that when comparing tunnel barrier layers being oxides formed by the plasma oxidation method and the natural oxidation method, the tunnel barrier layer formed by the natural oxidation method has a resistance value smaller than that of the tunnel barrier layer formed by the plasma oxidation method by no less than two figures. Accordingly, it can be the that the natural oxidation method is a quite preferable oxidation method when considering an application to the HDD head. However, the natural oxidation method also has some problems. Specifically, the first problem is that no less than one oxidation hour is required for forming a barrier layer so that the productivity is low. The second problem is that after an oxide film is formed on the surface of a non-magnetic metal layer made of, for example, Al, subsequent oxidation does not progress readily due to a poor oxidation force so that the so-called "oxidation residue" is generated inside the non-magnetic metal layer. This "oxidation residue" works as impurities to extremely deteriorate the TMR effect.
The present invention has been made under these circumstances and has an object to provide a ferromagnetic tunnel magnetoresistance effect element which is excellent in productivity and quality stability and highly excellent in TMR effect, and a method of producing such a ferromagnetic tunnel magnetoresistance effect element.